I. Field of the Invention
This invention relates to the field of electro-surgical medical devices. More particularly, this invention relates to devices that deliver energy in the form of radio-frequency electrical current to tissue in order to perform surgical functions.
II. Description of Related Art
Various medical procedures rely on high-frequency electrical currents to deposit energy and thus heat human and animal tissues. During such procedures, a high-frequency current is passed through the tissue between electrodes. One electrode is located at the tip of a surgical probe. Another electrode is located elsewhere, and may be a grounding pad or another surgical probe tip. The tissue to be treated lies between the electrodes.
When the electrode circuit is energized, the electric potential of the electrodes at the probe tips oscillates at radio frequencies about a reference potential. If one is used, a grounding pad remains at a floating reference potential. As the electric potential of the probe electrodes varies, a motive force on charged particles in the tissue is established that is proportional to the gradient of the electric potential. This electromotive force causes a net flow of electric charge, a current, to flow from one electrode, through the tissue, to any other electrode(s) at a lower potential. In the course of their flow, the charged particles collide with tissue molecules and atoms. This process acts to convert electrical energy to sensible heat in the tissue and is termed Joule heating.
Upon heating, surgical functions such as cutting, cauterizing and tissue destruction can be accomplished. For example, tissues can be cut by heating and eventually vaporizing the tissue cell fluids. The vaporization causes the cell walls to rupture and the tissue to cleave. When it is beneficial to destroy tissue, comparatively higher rates of energy deposition can cause tissue ablation.
Ablation of cellular tissues in situ is used in the treatment of many diseases and medical conditions either alone or combined with surgical removal procedures. Surgical ablation is often less traumatic than surgical removal procedures and may be the only alternative where other procedures are unsafe.
Tissue ablation devices commonly utilize electromagnetic (microwave, radio frequency (RF), lasers) or mechanical (acoustic) energy. In the category of electro-surgical devices, microwave ablation systems utilize a microwave antenna which is inserted into a natural body opening through a duct to the zone of treatment. Electromagnetic energy then radiates from the antenna through the duct wall into the target tissue. However, there is often severe trauma to the duct wall in this procedure since there is a significant microwave energy flux in the vicinity of the intended target. The energy deposition is not sufficiently localized. To reduce this trauma, many microwave ablation devices use a cooling system. However, such a cooling system complicates the device and makes it bulky. Laser ablation devices also suffer the same drawback as microwave systems. The energy flux near the target site, while insufficient to ablate the tissue, is sufficient to cause trauma.
Application of RF electric currents emanating from electrode tips offers the advantage of greater localization of the energy deposition since the electrode tip is nearly a point source. However, these devices require consideration and monitoring of the power applied to the tissue as well as the tissue response. Since the electric energy flux is localized, the electrical dissipation and storage characteristics of the tissue carrying the current may vary with time as a result of the current-induced heating. As a result, the power absorbed by the tissue as heat could vary over the time of treatment due to changing values of the tissue's electrical properties.
In addition, the localization of energy flux in an RF electro-surgical device may require a number of electrodes to be included in the surgical probe to provide adequate area coverage. With multiple electrodes in a surgical probe, each probe electrode may not be at the same electric potential at each instant due to amplitude, frequency or phase variations in their RF oscillations. In this case, an electric current would flow between the probe electrodes, coupling them to an extent primarily determined by the difference in electric potential between the electrodes and the electrical properties of the tissue between the electrode tips. Thus, the power may be delivered across several electrodes.
As described, electric power determination is critical in each electrode circuit of an RF electro-surgical device since it is directly related to the intended medical effects. Prior art approaches for determining the power on an electrode circuit utilize high speed analog multipliers to multiply measured current and voltage signals. A drawback to these approaches is that high speed, high-precision analog multipliers and associated root mean square (RMS) converters are expensive.
Prior art methods for RF waveform synthesis in electro-surgical devices often produce square waveforms repeating at radio frequencies. This approach, however, has the drawback that substantial filtering must be applied to remove the high-frequency Fourier components of the RF squarewave. This is necessary to comply with FCC regulations on emitters. The required filtering, typically achieved with a resonant inductor-capacitor (LC) circuit, degrades the control of the relative voltages at the electrode tips by requiring a sharp bandpass filter (a filter with high quality factor, Q). With a high Q filter, small differential variations in the tuning of the electrode channels (due, for example, to aging of the capacitors and inductors) lead to differential voltages at the electrode tips. As described, this can confuse monitoring of the power applied to the surgical site by inducing electrode coupling, termed cross-talk. Therefore, to improve control of the electric power applied to the patient, there is a need in the field for an improved method and apparatus for power measurement and radio frequency waveform synthesis in electro-surgical generators.